... At 21, you're not fully formed yet! I can almost guarantee that whatever relationship you're in now will NOT be the relationship you'll be in 5 years from now if that helps you relax a little. You have a whole lifetime ahead of you for seducing and being seduced...

While I agree that "you have a while lifetime ahead of you for seducing and being seduced" it is possible that someone might be in a relationship with the same person 5 years down the road (even at 21). The truth is that most potential relationships don't seem to weather the early stages...or, even if they do, they may not weather the fading of NRE down the road. (Which is why we have "dating" and "engagement" - so we can try things!)

Now, even if someone is in a relationship with the same person the relationship itself will be different - because people (and therefore the relationships that they are in) change and grow and evolve over time.

For the record, I am still with my first boyfriend ever (now my husband) - which comes as quite a surprise since I had never (until then) imagined myself "partnered" at all . We met when I was 18, married when I was 22...and that was 17 years ago. On the other hand, we were never, strictly speaking, monogamous (by agreement, although our boundaries have flexed and changed over time - evolving to where we are now)

The problem, as I see it, is not that you are young or clingy - but that the two of you are not in the same "place" in terms of what you want. Which isn't to say it can't work at all (MrS had to wait 6 mos for me to "catch on" to the fact that we were even dating) - but you can't force feelings, and you have to be able to accept that he may never be in a place to want a deeper relationship with you. Which is sad, but not catastrophic if you are able to allow yourself to be open to other opportunities at the same time.